As the bandwidth of a telecommunication signal increases, inter-symbol interference caused by frequency selectivity of a radio channel becomes a limiting factor when considering the reliability of a data transfer. The frequency selectivity is caused by multipath propagation of a telecommunication signal in the radio channel resulting in arrival of different transmitted signal components at a radio receiver at different time instants. The time difference between the time instants at which the first and the last signal components arrive at the receiver is called a delay spread. When the delay spread is higher than a symbol duration, inter-symbol interference occurs. In order to mitigate the effects of inter-symbol interference on reliable data detection, several types of equalizers have been developed.
An equalizer based on a “zero forcing” algorithm is a widely used, low-complexity equalizer which aims to eliminate the inter-symbol interference at decision time instants, i.e. at the center of the bit or symbol interval. The zero forcing equalizer may be realized, for example, with a transversal digital filter whose filter coefficients are calculated from the radio channel impulse response such that the output of the filter has a value (unity, for example) at the decision time instant and zero otherwise. The zero forcing equalizer actually attempts to counter the frequency selectivity of the radio channel by compensating the attenuation of the telecommunication signal at certain frequencies. This leads to a drawback associated with the zero forcing equalizer. If the radio channel frequency response has spectral nulls in its frequency spectrum, the zero forcing equalizer gives an amplification rising to infinity to these frequencies. This results in drastic amplification of noise at these frequencies and, thus, degradation in the performance of the equalizer.
Frequency domain equalizers transform a received telecommunication signal and a radio channel impulse response into a frequency domain. The frequency spectrum of the telecommunication signal is then weighted in the frequency domain with the frequency spectrum of the radio channel in order to compensate for the frequency attenuations caused by the radio channel. There exist a number of algorithms for carrying out the weighting. A problem with the frequency domain equalizers is that they are complex equalization methods consuming high amounts of computational resources. In particular, the frequency domain equalizers calculate a high amount of divisions which consume a substantial amount of computational resources particularly in ASIC (Application Specific Integrated Circuit) implementations in which the silicon area is a very limited resource.